Semiconductor memory device with a memory cell power supply circuit

ABSTRACT

A semiconductor memory device, including a memory cell including a flip-flop, and a memory cell power supply circuit for supplying a cell power supply voltage to the memory cell, wherein the memory cell power supply circuit supplies a cell power supply voltage in a first period and a different cell power supply voltage in a second period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device including flip-flop-type memory cells and, more particularly, to a technique for controlling the memory cell power supply.

2. Description of the Background Art

As the process rule becomes finer and finer in recent years, the circuit area and the power supply voltage of semiconductor integrated circuits have been reduced rapidly. However, with a semiconductor memory device including flip-flop-type memory cells, such as a static-type random access memory (SRAM), for example, such advancement makes the characteristics variations between transistors of the memory cells more significant. Due to such characteristics variations and to the reduced power supply voltage, it is now very difficult to realize stable memory cell characteristics, thereby lowering the production yield of semiconductor memory devices.

FIG. 8 shows an SRAM memory cell of a flip-flop type, which is formed by ordinary CMOS transistors. In FIG. 8, QN1 and QN2 are drive transistors, QN3 and QN4 are access transistors, QP1 and QP2 are load transistors, WL is a word line, BL and /BL are bit lines, and VDD is the power supply.

The load transistor QP1 and the drive transistor QN1 together form an inverter, and the load transistor QP2 and the drive transistor QN2 together form another inverter. The input/output terminals of the inverters are connected together to thereby form a flip-flop. The gate terminals of the access transistors QN3 and QN4 are connected to the word line WL, and the drain terminals thereof are connected to the bit lines BL and /BL, respectively. The source terminals of the access transistors QN3 and QN4 are connected to the input/output terminals of the inverters.

Data is written to the SRAM memory cell of FIG. 8 as follows. The bit lines BL and /BL are pre-charged to an H level. Then, the word line WL is brought from an L level to the H level (active) while the potential of one of the bit lines BL and /BL is brought from the H level to the L level.

Common memory cell characteristics of an SRAM include the write level and the static noise margin.

The write level represents the voltage with which data is written to the memory cell. Data is written to an SRAM memory cell by inverting the state of the flip-flop of the memory cell (note however that the state of the flip-flop is not inverted when the write data happens to be the same as the stored data). The critical bit line potential with which the state of the flip-flop of the memory cell can be inverted is referred to as the write level.

A lower write level gives a larger margin for erroneous writing (static noise margin) due to bit line noise, or the like, but presents a problem in that the flip-flop cannot be inverted until the bit line potential becomes sufficiently low, thereby resulting in a long write time. A higher write level gives a shorter write time, but also gives a smaller margin for erroneous writing (static noise margin).

A lower write level also means that the state of the flip-flop of the memory cell is less easily inverted due to bit line noise, or the like, in a read operation period, i.e., a larger static noise margin, and a higher write level also means that the state of the flip-flop of the memory cell is more easily inverted in a read operation period, i.e., a smaller static noise margin.

Thus, there is a trade-off between the write level and the static noise margin.

In order to solve such a problem, Japanese Laid-Open Patent Publication No. 55-64686, for example, discloses a technique for improving the write level by controlling the power supply voltage of the memory cell.

As the process rule becomes finer, there is a problem of an increased leak current flowing through transistors being OFF. Particularly, the memory cell section is often formed by transistors with small gate widths in order to reduce the area of the semiconductor memory device. A transistor with a smaller gate width, as compared with a transistor with a larger gate width, has a larger off-leak current per unit gate width.

The off-leak current flowing through a memory cell also varies depending on the arrangement of the memory cell array, the transistor characteristics (different impurity concentrations), the operating power supply voltage, the temperature condition, etc.

For example, in a write operation in a case where the power supply of a memory cell is controlled, the power supply voltage of memory cells connected to non-selected bit lines, other than the selected bit line to which data is to be written, will decrease unless there is supplied a charge sufficient to compensate for the amount of charge to be lost by the off-leak current flowing through the memory cells connected to the non-selected bit lines. Similarly, in a non-write operation period (in a read operation period), the power supply voltage of the memory cells connected to all bit lines will decrease unless there is supplied a charge sufficient to compensate for the amount of charge to be lost by the off-leak current flowing through the memory cells connected to the bit lines. If the power supply voltage of a memory cell decreases, the static noise margin of the memory cell deteriorates, whereby data in the memory cell is more likely to be corrupted (inverted).

In the configuration disclosed in FIG. 2 of Japanese Laid-Open Patent Publication No. 55-64686, the power supply voltage of the memory cell is set to a level lower than the VDD level in a non-write operation period (in a read operation period). Therefore, as described above, the static noise margin of the memory cell deteriorates, and corruption of memory cell data is more likely to occur.

In the configuration disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 55-64686, no countermeasures are taken for the very high leak current level flowing through the memory cell, which occurs in semiconductor memory devices of the latest generation. In a write operation period, if there is a very high off-leak current through memory cells connected to non-selected bit lines other than the selected bit line to which data is to be written, there is also a substantial drop in the memory cell power supply voltage caused by the off-leak current. Therefore, the static noise margin of the memory cell deteriorates, and corruption of memory cell data is more likely to occur.

Moreover, in the configuration disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 55-64686, since the output of the inverter including a depletion transistor and an enhancement transistor connected in series together is supplied as the power supply of the memory cell, there is always a through current in a write operation period. Moreover, the configuration employs the depletion transistor in order to compensate for the amount of charge to be lost by the off-leak current flowing through the memory cells connected to the non-selected bit lines. However, as the process rule becomes finer, there is a problem of an increased leak current flowing through transistors being OFF, as described above. Therefore, in order to compensate for the amount of charge to be lost by the off-leak current, it is necessary to employ a depletion transistor with a sufficiently large driving capability. This in turn requires an increase in the driving capability of an enhancement transistor in order to sufficiently lower the memory cell power supply. With the approach disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 55-64686, the through current flowing through the inverter including a depletion transistor and an enhancement transistor connected in series together increases, thereby increasing the power consumption. Another problem is that increasing the driving capability of the transistors will increase the layout area.

Japanese Laid-Open Patent Publication No. 55-64686 also fails to take measures to optimally compensate for the leak current depending on the arrangement of the memory cell array, the transistor characteristics, the operating power supply voltage, the temperature condition, etc. Therefore, unless the power supply of the memory cell is controlled appropriately depending on the arrangement of the memory cell array, the transistor characteristics, the operating power supply voltage, the temperature condition, etc., the power consumption and the layout area will increase.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a semiconductor memory device in which it is possible to optimally control the memory cell power supply depending on the arrangement of the memory cell array, the transistor characteristics, the operating power supply voltage, the temperature condition, etc.

In view of the above, the present invention provides a semiconductor memory device, including a memory cell including a flip-flop, and a memory cell power supply circuit for supplying a cell power supply voltage to the memory cell, wherein the memory cell power supply circuit supplies a cell power supply voltage in a first period and a different cell power supply voltage in a second period.

For example, the memory cell power supply circuit supplies a predetermined first power supply voltage in a case where the cell power supply voltage is supplied in a data read cycle and in a case where data is not written to a memory cell to which the cell power supply voltage is supplied in a write cycle, while the memory cell power supply circuit supplies a second power supply voltage lower than the first power supply voltage in a case where data is written to a memory cell to which the cell power supply voltage is supplied in a write cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a semiconductor memory device of Embodiment 1.

FIG. 2 is a circuit diagram showing a configuration of a memory cell power supply control circuit of Embodiment 2.

FIG. 3 is a circuit diagram showing a configuration of a memory cell power supply control circuit of Embodiment 3.

FIG. 4 is a circuit diagram showing a configuration of a memory cell of Embodiment 4.

FIG. 5 is a circuit diagram showing a configuration of a memory cell power supply control circuit of Embodiment 5.

FIG. 6 is a circuit diagram showing another configuration of the memory cell power supply control circuit of Embodiment 5.

FIG. 7 is a circuit diagram showing a configuration of a semiconductor memory device including a redundant circuit block of Embodiment 6.

FIG. 8 is a circuit diagram showing a configuration of a conventional semiconductor memory device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a semiconductor memory device of the present invention, it is possible to optimally control the memory cell power supply depending on the arrangement of the memory cell array, the transistor characteristics, the power supply voltage, the temperature condition, etc. Particularly, the memory cell write level is improved, whereby it is possible to realize a semiconductor memory device having stable memory cell characteristics with a low power consumption and a small area.

Preferred embodiments of the present invention will now be described with reference to the drawings. Note that in each of the following embodiments, like elements to those of any preceding embodiments will be denoted by like reference numerals, and will not be described repeatedly.

Embodiment 1

FIG. 1 shows a configuration of a semiconductor memory device according to Embodiment 1 of the present invention. Circuits for reading data are omitted in the figure for the sake of simplicity.

The semiconductor memory device shown in FIG. 1 includes memory cells 1, precharge circuits 2, memory cell power supply control circuits 3, and leak compensation circuits 4. In FIG. 1, QN1 to QN2 are drive transistors, QN3 to QN4 are access transistors, QN5 to QN8 are N-type MOS transistors, QP1 to QP2 are load transistors, QP3 to QP4 are precharge transistors, QP5 is an equalization transistor, QP6 to QP8 are P-type MOS transistors, INV1 to INV2 are inverters, AND1 to AND4 are 2-input AND circuits, NAND1 to NAND2 are 2-input NAND circuits, WL1 to WL2 are word lines, BL1 to BL2 and /BL1 to /BL2 are bit lines, PCG is a precharge control signal, AD0 to AD1 are column address signals, DIN and /DIN are input data, WEN is a write enable control signal, VDDM1 to VDDM2 are memory cell power supplies, and VDD is the power supply.

Each memory cell 1 is provided at the intersection between the word line WL1 or WL2 extending in the row direction and the bit line BL1, /BL1 or BL2, /BL2 extending in the column direction. The memory cells 1 arranged in a matrix pattern together form a memory array section for storing information. In the memory cell 1, the load transistor QP1 and the drive transistor QN1 together form an inverter, and the load transistor QP2 and the drive transistor QN2 together form another inverter. The input/output terminals of the inverters are connected together to thereby form a flip-flop. The gate terminals of the access transistors QN3 and QN4 are connected to the word line WL1 (WL2), and the drain terminals are connected to the bit lines BL1 and /BL1 (BL2 and /BL2). The source terminals of the access transistors QN3 and QN4 are connected to the input/output terminals of the inverters. The source terminals of the load transistors QP1 and QP2 forming an inverter are connected to the memory cell power supply VDDM1 (VDDM2) being the output of the memory cell power supply control circuit 3.

Each precharge circuit 2 is provided at the intersection between the precharge control signal PCG extending in the row direction and the bit line BL1, /BL1 or BL2, /BL2 extending in the column direction. The precharge circuit 2 is formed by the precharge transistors QP3 and QP4 and the equalization transistor QP5, and the gate terminals of these transistors are connected to the precharge control signal PCG. The source terminals of the precharge transistors QP3 and QP4 are connected to the power supply VDD, and the drain terminals are connected to the source terminal and the drain terminal of the equalization transistor QP5. The drain terminals of the precharge transistors QP3 and QP4 are also connected to the bit lines BL1 and /BL1 (BL2 and /BL2).

In the precharge circuit 2, the precharge control signal PCG is at the L level when the word line is inactive (the L level), thereby precharging the bit lines to the H level. If one of the word lines is active (the H level), the precharge control signal PCG is at the H level, whereby all the P-type MOS transistors QP3 to QP5 of the precharge circuit 2 are turned OFF, thereby achieving a state (the high impedance state) where there is no influence on the bit lines.

The memory cell power supply control circuit 3 is provided for each column to provide the memory cell power supply VDDM1 (VDDM2) to the memory cells 1 provided along the same bit line. The memory cell power supply control circuit 3 includes the P-type MOS transistor QP6 whose source terminal is connected to the power supply VDD and whose gate terminal is grounded, and the P-type MOS transistor QP7 whose source terminal is connected to the drain terminal of the P-type MOS transistor QP6 and whose drain terminal is grounded. The connection node between the drain terminal of the P-type MOS transistor QP6 and the source terminal of the P-type MOS transistor QP7 is output as the memory cell power supply VDDM1 (VDDM2). The gate terminal of the P-type MOS transistor QP7 is connected to the output node of the 2-input NAND circuit NAND1 (NAND2) receiving the column address signal AD0 (AD1) and the write enable control signal WEN. In the memory cell power supply control circuit 3, the P-type MOS transistor QP6 being always ON and the P-type MOS transistor QP7 controlled by the column address signal and the write enable control signal are connected in series together, thereby forming a source follower.

The memory cell power supply control circuit 3 operates as follows when writing data to the memory cell 1 connected to a bit line being selected by the column address signal. For example, when the column address signal AD0 is at the H level (AD1 is at the L level) and the write enable control signal WEN is at the H level, the output of the 2-input NAND circuit NAND1 is brought to the L level, and the P-type MOS transistor QP7 is turned ON, whereby the power supply of the memory cell 1 connected to the bit lines BL1 and /BL1 (the memory cell power supply VDDM1) is forcibly controlled to a voltage value lower than the VDD level that is determined by the voltage divider ratio between the P-type MOS transistors QP6 and QP7.

In a write operation, the voltage level of the memory cell power supply output from the memory cell power supply control circuit 3 is determined depending on the power supply VDD and the voltage divider ratio (strictly speaking, it is also dependent on the cell leak). Then, the write level of the memory cell 1 is determined depending on the voltage level of the memory cell power supply (strictly speaking, it is also dependent on the threshold voltage of the load transistors QP1 and QP2 of the memory cell 1). Therefore, it is possible to realize a desirable write level by appropriately setting the voltage divider ratio, whereby it is possible to easily realize both the ease of data writing and a desirable write margin.

In a case where the write enable control signal WEN is at the L level or the column address signal is at the L level (i.e., a case where the power supply is provided to the memory cell 1 when no data is being written, or a case where data is being written and the power supply is provided to the memory cells 1 connected to non-selected bit lines, other than the bit line being selected by the column address signal), the output of the 2-input NAND circuit NAND1 (NAND2) is at the H level, and the P-type MOS transistor QP7 is turned OFF, whereby the memory cell power supply VDDM1 (VDDM2) is controlled to be at the VDD level by the P-type MOS transistor QP6. Thus, a charge is supplied in a manner similar to that of the leak compensation circuit 4 to be described later.

One leak compensation circuit 4 is provided for each column, and includes the P-type MOS transistor QP8. The source terminal of the P-type MOS transistor QP8 is connected to the power supply VDD, and the drain terminal is connected to the memory cell power supply VDDM1 (VDDM2). The gate terminal of the P-type MOS transistor QP8 is connected to the output node of the inverter INV1 (INV2), which receives the output of the 2-input NAND circuit NAND1 (NAND2) receiving the column address signal AD0 (AD1) and the write enable control signal WEN. Thus, the gate terminal of the P-type MOS transistor QP8 receives the logical product between the column address signal and the write enable control signal WEN.

Data is written to the memory cell 1 connected to the bit line being selected by the column address signal (the write enable control signal WEN at the H level) as follows, in a case where, for example, the column address signal AD0 is at the H level (AD 1 at the L level) and the write enable control signal WEN is at the H level. The gate input of the P-type MOS transistor QP8 of the leak compensation circuit 4 corresponding to the bit lines BL1 and /BL1 being selected by the column address signal is at the H level, and the P-type MOS transistor QP8 is turned OFF, whereby there is no influence on the memory cell power supply VDDM1. On the other hand, the gate input of the P-type MOS transistor QP8 of the leak compensation circuit 4 corresponding to the non-selected bit lines BL2 and /BL2 is at the L level, and the P-type MOS transistor QP8 is turned ON, whereby the VDD level is supplied to the memory cell power supply VDDM2.

In a non-write operation period (in a read operation period), the write enable control signal WEN is at the L level. Therefore, for all the leak compensation circuits 4, the gate input of the P-type MOS transistor QP8 is at the L level and the P-type MOS transistor QP8 is turned ON, whereby the VDD level is supplied to all the memory cell power supplies VDDM1 and VDDM2.

As described above, in a write operation period, the leak compensation circuit 4 does not influence the memory cells 1 along the selected bit line while supplying the VDD level to the memory cell power supply for the memory cells 1 along each non-selected bit line. In a non-write operation period (in a read operation period), the VDD level is supplied to the memory cell power supplies for the memory cells 1 along all bit lines.

The N-type MOS transistors QN5 to QN8 each operate as a write buffer circuit for writing data to the memory cells 1. The drain terminals of these transistors are connected to the bit lines BL1, /BL1, BL2 and /BL2, and the source terminals are grounded. The gate terminals receive the output signals from the 2-input AND circuits AND1 to AND4, each being a logical product between the column address signal AD0 or AD1 and the input data DIN or /DIN.

For example, data is written as follows to the memory cell 1 provided at the intersection between the word line WL1 and the bit lines BL1 and /BL1.

First, the precharge control signal PCG is brought to the L level, and all the bit lines are brought to the precharged state (the H level). Then, when the precharge control signal PCG is brought to the H level, the precharged state of the bit lines is canceled, whereby the word line WL1, for example, is brought to the H level (active) (the word line WL2 is at the L level). Then, the column address signal AD0, for example, is brought to the H level (the column address signal AD1 is at the L level), and the H level is input as the input data DIN (the input data /DIN is at the L level), for example. Since the column address signal AD0 is at the H level and the input data DIN is at the H level, only the 2-input AND circuit AND2 outputs the H level (the 2-input AND circuits AND1, AND3 and AND4 output the L level), and only the N-type MOS transistor QN6, among other write buffer circuits, is turned ON. Therefore, only the bit line /BL1 is at the L level, thus allowing for a data write operation to the memory cell 1, to which data is to be written.

By lowering the power supply of the flip-flop forming a memory cell (the memory cell power supply) from the VDD level, the voltage at which data can be written to the memory cell is improved (the write level of the memory cell is increased) in a data write operation for the memory cell, and there are also improvements on the lower limit operating voltage of the semiconductor memory device and the speed at which data is written to the memory cell.

Our evaluation shows that if the memory cell power supply is lowered by 150 mV where the power supply voltage VDD is 1.0 V for a memory cell of a 65 nm CMOS process, the lower limit operating voltage for a write operation is improved by about 200 mV and the speed at which data is written to the memory cell is improved by about 100 ps.

Therefore, by employing a configuration such that the power supply of the memory cells connected along the selected bit line, to which data is to be written, is lowered by the memory cell power supply control circuit 3 in a write operation period, as in the present embodiment, it is possible to improve the lower limit operating voltage of the memory cell for a write operation (the write level of the memory cell) and to improve the write speed.

As the process rule becomes finer, there is a problem of an increased leak current flowing through transistors being OFF in the memory cell. As the number of memory cells connected to each bit line increases, the leak current increases, and the voltage drop in the memory cell power supply voltage also increases.

If the power supply of a non-selected memory cell connected along a bit line, to which data is not to be written, is lowered from the VDD level, in a write operation period, due to the voltage drop caused by the leak current through memory cells along the same bit line, the static noise margin of the memory cell deteriorates. Our evaluation shows that if there is a voltage drop of 20 mV from the VDD level in a memory cell of a 65 nm CMOS process where the power supply voltage VDD is 1.0 V, the static noise margin deteriorates by about 10 mV.

On the other hand, by employing a configuration such that the amount of charge to be lost through the memory cell leak current can be compensated for by the leak compensation circuit 4 in a write operation period or in a non-write operation period (in a read operation period), as in the present embodiment, it is possible to improve the static noise margin of the memory cell.

As described above, it is possible to improve the write level while maintaining the static noise margin characteristics of the memory cell, whereby it is possible to realize a semiconductor memory device with stable memory cell characteristics.

Embodiment 2

Assume a case where due to process variations, or the like, the characteristics of the N-type MOS transistors in Embodiment 1 fluctuate, whereby the threshold voltage of the N-type MOS transistors changes. For example, if the threshold voltage of N-type MOS transistors increases, the threshold voltage of the access transistors QN3 and QN4 and the drive transistors QN1 and QN2, being N-type MOS transistors among all the transistors of the memory cell 1, increases. If the threshold voltage of the N-type MOS transistors increases, the static noise margin of the memory cell increases, but the write level of the memory cell decreases, thereby making it more difficult to write data to the memory cell. Then, in order to make it easier to write data to the memory cell, the memory cell power supply can be further reduced.

In view of this, the P-type MOS transistor QP6, which is always ON, of the memory cell power supply control circuit 3 of FIG. 1 is replaced by the N-type MOS transistor QN9, which is always ON, whose gate terminal and whose source terminal are both connected to the power supply VDD, as shown in FIG. 2. If the threshold voltage of the N-type MOS transistors increases, the threshold voltage of the replacing N-type MOS transistor QN9 also increases.

Since the driving capability of the N-type MOS transistor QN9, which is always ON, is decreased while maintaining the driving capability of the P-type MOS transistor QP7, the level of the memory cell power supply, which is determined by the voltage divider ratio between the N-type MOS transistor QN9 and the P-type MOS transistor QP7, decreases. If the threshold voltage of the N-type MOS transistors decreases, the level of the memory cell power supply increases.

Thus, using a transistor of the same polarity as that of the access transistors or the drive transistors, the P-type MOS transistor QP6 of the memory cell power supply control circuit 3 of FIG. 1 is replaced by the N-type MOS transistor QN9, whereby it is possible to easily control the memory cell power supply while following changes in the threshold voltage of the access transistors or the drive transistors.

Embodiment 3

In Embodiment 1, transistors of the memory cell power supply control circuit 3 are all P-type MOS transistors (FIG. 1). If the purpose is solely to provide a memory cell power supply, a P-type MOS transistor and an N-type MOS transistor may be connected in series together, or two N-type MOS transistors may be connected in series together.

However, actual transistors have variations in the transistor characteristics, due to process variations, etc. Particularly, where a P-type MOS transistor and an N-type MOS transistor are combined with each other, if their transistor characteristics vary in the opposite directions (e.g., if the driving capability of the P-type MOS transistor is decreased while that of the N-type MOS transistor is increased), such variations in the transistor characteristics have a greater influence, as compared with a case where MOS transistors of the same polarities are used. Therefore, if the memory cell power supply control circuit 3 is formed only by transistors of the same polarity, it is possible to reduce the possible influence of the transistor characteristics variations.

For example, the P-type MOS transistor QP6, which is always ON, of the memory cell power supply control circuit 3 of FIG. 1 may be replaced by the N-type MOS transistor QN9, which is always ON, whose gate terminal and whose source terminal are both connected to the power supply VDD, and the P-type MOS transistor QP7, which is connected in series with the N-type MOS transistor QN9 in FIG. 2 can be replaced by the N-type MOS transistor QN10, as shown in FIG. 3. As the P-type MOS transistor QP7 shown in FIG. 2 is replaced by the N-type MOS transistor QN10, the gate terminal input of the N-type MOS transistor QN10 can be switched from the output node of the 2-input NAND circuit NAND1 (NAND2) to the output node of the inverter INV1 (INV2).

As described above, if the memory cell power supply control circuit 3 is formed only by transistors of the same polarity, it is possible to reduce the influence caused by the difference between the characteristics variations of P-type transistors and those of N-type transistors in the memory cell power supply control circuit 3. Therefore, it is possible to supply a stable memory cell power supply voltage against such a difference in characteristics variations.

Embodiment 4

It is preferred that the substrate nodes of the load transistors QP1 and QP2 of the memory cell 1 are connected to the VDD power supply (to separate the source nodes of the load transistors QP1 and QP2 from the substrate nodes) as shown in FIG. 4. In a write operation period, the memory cell power supply of the memory cell 1 connected to the selected bit line decreases from the VDD level as described above. If the substrate nodes of the load transistors QP1 and QP2 are at the VDD level, there will be a substrate bias effect for the load transistors QP1 and QP2, whereby the absolute value of the threshold voltage of the load transistors QP1 and QP2 increases and the driving capability decreases. Therefore, in a write operation, it is possible to more easily write data to the memory cell (the write level is improved). The increase in the absolute value of the threshold voltage of the load transistors QP1 and QP2 decreases the leak current flowing through memory cells that are connected to the selected bit line and are connected to word lines that are not being active, whereby it is possible to reduce the power consumption.

Embodiment 5

Moreover, the static noise margin of the memory cell can be improved at the same time by controlling the power supply of the memory cells connected to non-selected bit lines to be greater than or equal to the VDD level in a write operation period, while controlling the power supply of all the memory cells to be greater than or equal to the VDD level in a non-write operation period (in a read operation period). Specifically, this can be achieved easily by replacing the level of the power supply VDD received by the source terminal of the P-type MOS transistor QP8 forming the leak compensation circuit 4 of FIG. 1 with a level greater than or equal to the VDD level. A level greater than or equal to the VDD level can be applied by providing a booster circuit capable of generating a voltage greater than or equal to the VDD level inside the SRAM macro, or may be supplied from outside the SRAM macro.

Regarding Output Voltage, Driving Capability, Etc., of Memory Cell Power Supply Control Circuit 3

The output voltage (the voltage divider ratio of the P-type MOS transistors QP6 and QP7, etc), the driving capability, etc., of the memory cell power supply control circuit 3 in the semiconductor memory device as described above will now be discussed.

If the characteristics, the driving capability, the size and the shape of transistors forming a memory cell, and the number of memory cells to be connected along the same bit line vary, the write level of the memory cell and the static noise margin thereof will vary, and the leak current flowing through the memory cell will also vary. The various characteristics parameters may also differ depending on the power supply voltage and the temperature condition during operation. Moreover, if the transistor size of the memory cell, the memory cell layout arrangement, the length of the memory cell in the bit line direction, etc., vary, the bit line load capacitance will vary, and the speed at which data is written to the memory cell will also vary. It is understood that it is preferable to provide the memory cell power supply control circuit 3 and the leak compensation circuit 4 capable of accommodating the various characteristics differences as described above.

At present, such circuit design accommodating the characteristics variations can be easily done by performing a high-precision circuit simulation.

Determination of Voltage Divider Ratio

The voltage divider ratio of the P-type MOS transistors QP6, QP7, etc., can be determined so that the write operation reliably can be completed within a predetermined period of time by the power supply voltage output from the memory cell power supply control circuit 3 during a write operation, while it is possible to obtain a sufficiently large margin for erroneous writing. Specifically, it is done as follows.

For example, the write level of a memory cell varies as the temperature of the semiconductor memory device changes. Specifically, when the temperature decreases, the threshold voltage of the transistors of the memory cell increases, whereby the write level of the memory cell decreases, and it is more difficult to write data to the memory cell. In view of this, the voltage of the memory cell power supply to be output according to the voltage divider ratio of the P-type MOS transistors QP6 and QP7 forming the memory cell power supply control circuit 3 can be determined according to the temperature of use. Specifically, for a memory to be used at a lower temperature, a lower voltage can be used as the memory cell power supply voltage so that the write level increases, and it is easier to write data to the memory cell.

On the other hand, if a semiconductor memory device is used under a high temperature condition, the threshold voltage of the transistors of the memory cell decreases, whereby the write level of the memory cell increases, and it is easy to write data to the memory cell. Therefore, the memory cell power supply can be set to a higher level than that of a semiconductor memory device to be used under a low temperature condition.

If the memory cell size varies, the characteristics, for example, of the transistors forming the memory cell vary. Therefore, the write level of the memory cell differs for each SRAM macro. For example, if there are an SRAM macro in which the write level of the memory cell is low and an SRAM macro in which the write level is high, the memory cell power supply of the SRAM macro for which it is easier to write data to the memory cell (the one with a higher write level) can be set to a higher level than that of the SRAM macro for which it is less easy to write data to the memory cell (the one with a lower write level).

If the operating power supply voltage varies between SRAM macros, the memory cell write level varies therebetween. Typically, an SRAM macro with a higher operating power supply voltage has a larger write margin, and it is easier to write data to the memory cell. As the operating power supply voltage of an SRAM macro is lower, the write margin is smaller, whereby it is more difficult to write data to the memory cell. Therefore, the memory cell power supply of an SRAM macro with a higher operating power supply voltage can be set to a higher level than that of an SRAM macro with a lower operating power supply voltage.

Moreover, if the threshold voltage of the transistors of the memory cell varies, the write level will vary from one SRAM macro to another, whereby settings similar to those described above are applicable.

Thus, in cases such as where the temperature of use is high, where it is easy to write data due to, for example, the transistor characteristics according to the memory cell size, and where an SRAM macro with a high operating power supply voltage or an SRAM macro with a low threshold voltage is used, the voltage divider ratio of the P-type MOS transistors QP6 and QP7 of the memory cell power supply control circuit 3 can be set so that the level of the memory cell power supply is high.

More specifically, the transistor driving capability of the P-type MOS transistor QP7 can be set to a low level. As the transistor driving capability of the P-type MOS transistor QP7 is lowered, it is also possible to reduce the through current flowing through the P-type MOS transistors QP6 and QP7 in a write operation period and to realize a low power consumption. Moreover, lowering the transistor driving capability allows for a reduction in the layout area.

In a case where the same substrate carries SRAM macros differing from one another in terms of the ease of writing data dependent on the transistor characteristics, etc., according to the memory cell size in the SRAM macro, the operating power supply voltage, the threshold voltage, etc., the settings can be done so that the voltage divider ratio will differ from one another in view of these differences. At present, the scale of semiconductor integrated circuits such as system LSIs has become very large, and it is common to have semiconductor memory devices of different bit and word configurations formed on the same substrate. There are also products such as those called “multi-VT (threshold voltages)” in which transistor characteristics of different threshold voltages are provided on the same substrate so that a circuit block of a stringent speed requirement is formed by transistors of lower threshold voltages, whereas a circuit block for which a lower power consumption is more important than a higher speed is formed by transistors of higher threshold voltages. In such a case, the voltage divider ratios can be determined so that an appropriate memory cell power supply voltage is supplied to each SRAM macro.

Determination of Driving Capability

The driving capability of the P-type MOS transistors QP6 and QP7 can be determined according to the number of memory cells to which the power supply voltage is supplied, the required write speed, etc., so that it is possible to reliably complete a write operation within a predetermined period of time while it is possible to obtain a sufficient margin for erroneous writing. Specifically, it is done as follows.

For example, assume that the transistor driving capability of the P-type MOS transistors QP6 to QP8 of the memory cell power supply control circuit 3 and the leak compensation circuit 4 has been optimized for an SRAM in which 512 memory cells are connected along the same bit line. Assume that the optimization is directly applied to another SRAM in which 32 memory cells are connected along the same bit line.

The voltage level (the DC-wise voltage level) of the memory cell power supply, which is output based on the voltage divider ratio of the P-type MOS transistors QP6 and QP7 of the memory cell power supply control circuit 3, stays substantially constant against changes in the number of memory cells. Therefore, the memory cell power supply control circuit 3, which is designed for an application where the number of memory cells is 512, can be used in another application where the number of memory cells is 32. However, a change in the number of memory cells connected along the same bit line and a change in the bit line length of the memory cell mean a change in the load capacity of the memory cell power supply. In a case where the number of memory cells is 32, as compared with a case where it is 512, the load capacity of the memory cell power supply is smaller, whereby the voltage level (the dynamic voltage level) of the memory cell power supply changes more quickly than necessary. Rapid changes in the memory cell power supply may possibly cause corruption of data stored in memory cells. In this case, it is only necessary that the change in the memory cell power supply is completed within a period of time that is required for completing a write operation to a memory cell. Thus, the transistor driving capability of the P-type MOS transistors QP6 and QP7 of the memory cell power supply control circuit 3 can be lowered.

SRAM macros of different write speed requirements will be discussed below. If the required write speed is slow, it is only necessary that the change in the memory cell power supply is completed within a period of time that is required for completing a write operation to a memory cell. Thus, again, the transistor driving capability of the P-type MOS transistors QP6 and QP7 of the memory cell power supply control circuit 3 can be lowered.

Thus, the transistor driving capability can be lowered for an SRAM macro with a smaller number of memory cells or for an SRAM macro where the required write speed is slow. If the transistor driving capability is lowered, the through current flowing through the P-type MOS transistors QP6 and QP7 is reduced, whereby it is possible to reduce the power consumption of the semiconductor memory device. Moreover, lowering the transistor driving capability allows for a reduction in the layout area.

As described above with respect to the voltage divider ratio, the driving capability can be lowered in a case where the temperature of use is high, in a case where it is easy to write data due to, for example, the transistor characteristics according to the memory cell size, and in a case where an SRAM macro with a high operating power supply voltage or an SRAM macro with a low threshold voltage is used.

With respect to the number of memory cells, the write speed requirement, the memory cell size, the operating power supply voltage and the threshold voltage, if a plurality of SRAM macros differing from one another in terms of these parameters are provided on the same substrate, the driving capability of each SRAM macro can be determined to be different from those of the other SRAM macros in view of these parameters.

Inactivation of Memory Cell Power Supply Control Circuit 3

In a case where data can be written to a memory cell with no problems even without lowering the memory cell power supply from the VDD level (i.e., without increasing the write level of the memory cell), the gate input of the P-type MOS transistor QP7 of the memory cell power supply control circuit 3 may be controlled so as to be fixed at the H level, to thereby achieve a state where the memory cell power supply control circuit 3 is always inactive, i.e., a state where the memory cell power supply is always outputting only the VDD level by the P-type MOS transistor QP6 being always ON.

Specifically, cases where data can be written with no problems include, for example, a case where the number of memory cells connected along the same bit line is relatively small (each memory cell thus having a shorter bit line length), a case where the required write speed is relatively slow, a case where the memory cell size is small, a case where the operating power supply voltage is relatively high, and a case where the threshold voltage of transistors of the memory cell is relatively low.

Specifically, as shown in FIG. 5, a 2-input OR circuit OR1 (OR2), which is controlled by a control signal 1 and the output node of the 2-input NAND circuit NAND1 (NAND2), is inserted between the output node of the 2-input NAND circuit NAND1 (NAND2) and the gate terminal input of the P-type MOS transistor QP7 of the memory cell power supply control circuit 3 shown in FIG. 1. If the H level is input as the control signal 1, the output of the 2-input OR circuit OR1 (OR2) is at the H level, whereby the P-type MOS transistor QP7 whose gate terminal receives the output of the 2-input OR circuit OR1 (OR2) is turned OFF. Therefore, it is possible to easily control the memory cell power supply control circuit 3 to be always inactive. If the memory cell power supply control circuit 3 is controlled to be always inactive, there will be no through current flowing through the P-type MOS transistors QP6 and QP7 during a write operation period, whereby it is possible to reduce the power consumption.

Also with this configuration where the memory cell power supply control circuit 3 can be made inactive, if a plurality of SRAM macros differing from one another in terms of the number of memory cells connected along the same bit line (the bit line length of each memory cell), the write speed requirement, the memory cell size, the operating power supply voltage, or the threshold voltage of transistors of the memory cell, are provided on the same substrate, a configuration as shown in FIG. 5 may be used so that the memory cell power supply control circuit 3 can be controlled to be always inactive only for some SRAM macros depending on the difference in terms of the above parameters.

The memory cell power supply control circuit 3 may be controlled to be always inactive as described above depending on a predetermined temperature of use, or whether or not to control the memory cell power supply control circuit 3 to be inactive may be determined depending on the actual temperature of use.

Regarding Driving Capability, Etc., of Leak Compensation Circuit 4

Next, the driving capability, etc., of the leak compensation circuit 4 will be discussed.

In cases such as where the semiconductor memory device is used under a low temperature condition, the threshold voltage of transistors of the memory cell increases, whereby the leak current flowing through the memory cell decreases. If the leak current decreases, the driving capability of the P-type MOS transistor of the leak compensation circuit 4 can be lowered. Lowering the transistor driving capability allows for a reduction in the layout area.

With respect to the P-type MOS transistor QP8 of the leak compensation circuit 4, the P-type MOS transistor QP8 designed for an application where the number of memory cells is 512 can be used in another application where the number of memory cells is 32. As the number of memory cells connected along the same bit line is decreased from 512 to 32, the leak current through the memory cells also decreases. Therefore, the transistor driving capability is only required to be at a level such that it is possible to compensate for the leak current for 32 memory cells. Thus, it is possible to lower the transistor driving capability, and lowering the transistor driving capability allows for a reduction in the layout area.

In an SRAM macro with a high transistor threshold voltage, the leak current flowing through the memory cell is decreased, whereby it is possible to lower the driving capability of the P-type MOS transistor QP8 of the leak compensation circuit 4, and lowering the transistor driving capability allows for a reduction in the layout area.

In a case where there is only a slight leak current through the memory cell, and there is substantially no voltage drop in the memory cell power supply due to the leak current, there will be no deterioration in the static noise margin of the memory cell, and the leak compensation circuit 4 can therefore be omitted. In such a case, the gate input of the P-type MOS transistor QP8 of the leak compensation circuit 4 can be controlled to be fixed at the H level, whereby the leak compensation circuit 4 is always inactive.

Specifically, as shown in FIG. 6, the inverter INV1 (INV2) shown in FIG. 1 is replaced by a 2-input NAND circuit NAND3 (NAND4) so that one input of the 2-input NAND circuit NAND3 (NAND4) can be controlled by a control signal 2. If the L level is input as the control signal 2, the output of the 2-input NAND circuit NAND3 (NAND4) is at the H level, whereby the P-type MOS transistor QP8 whose gate terminal receives the output of the 2-input NAND circuit NAND3 (NAND4) is turned OFF. Therefore, it is possible to easily control the leak compensation circuit 4 to be always inactive.

Again, in a case where a plurality of SRAM macros differing from one another in terms of the number of memory cells connected along the same bit line (the bit line length of each memory cell), the write speed requirement, or the threshold voltage of transistors of the memory cell, are provided on the same substrate, the driving capability may be varied depending on these parameters, or a configuration such as that shown in FIG. 6 may be used only for some SRAM macros so that the leak compensation circuit 4 can be controlled to be always inactive.

As described above, if the memory cell power supply control circuit 3 and the leak compensation circuit 4 are configured so that they have optimal transistor characteristics depending on the various configurations and conditions, it is possible to easily reduce the power consumption or the area. Specifically, it is possible to easily realize both the ease of writing and a desirable write margin by controlling the characteristics of the memory cell power supply control circuit 3 and the leak compensation circuit 4 while the semiconductor memory device is being used by using a control signal produced inside the semiconductor integrated circuit, an external control signal, or the like, according to changes in the temperature, the power supply voltage or the operating frequency (the write speed requirement) and changes in the threshold voltage by the back-bias control, etc. Alternatively, when producing semiconductor memory devices of various specifications, configurations as shown in FIG. 5 and FIG. 6 can be used with various control signals according to the various specifications being fixedly input thereto. Then, by using the same SRAM macros, it is possible to produce a semiconductor memory device in which the memory cell power supply voltage varies and a semiconductor memory device in which it is constant. Thus, it is not necessary to develop different types of SRAM macros having different functions and different characteristics, and the development period can be easily shortened. This can be particularly effective for so-called “memory compliers”, and the like, capable of variably producing semiconductor memory devices of different bit and word configurations (address spaces).

Embodiment 6

FIG. 7 shows a configuration of a semiconductor memory device according to Embodiment 6 of the present invention. Circuits for reading data are omitted in the figure for the sake of simplicity.

The semiconductor memory device shown in FIG. 7 includes the memory cells 1, the precharge circuits 2, the memory cell power supply control circuits 3, the leak compensation circuits 4, a regular circuit block 5, and a redundant circuit block 6. In FIG. 7, QN1 to QN2 are drive transistors, QN3 to QN4 are access transistors, QN5 to QN8 and QNR5 to QNR8 are N-type MOS transistors, QP1 to QP2 are load transistors, QP3 to QP4 are precharge transistors, QP5 is an equalization transistor, QP6 to QP8 are P-type MOS transistors, INV3 is an inverter, AND5 to AND8 and ANDR5 to ANDR8 are 3-input AND circuits, NAND5 to NAND6 and NANDR5 to NANDR6 are 3-input NAND circuits, NAND7 to NAND8 and NANDR7 to NANDR8 are 2-input NAND circuits, WL1 to WL2 are word lines, BL1 to BL2 and /BL1 to /BL2 are bit lines, BLR1 to BLR2 and /BLR1 to /BLR2 are redundant bit lines, PCG is a precharge control signal, AD0 to AD 1 are column address signals, DIN and /DIN are input data, WEN is a write enable control signal, REDEN is a redundant signal, /REDEN is an inverted redundant signal, VDDM1 to VDDM2 are memory cell power supplies, VDDMR1 to VDDMR2 are redundant memory cell power supplies, VDD is the power supply. The memory cells 1, the precharge circuits 2, the memory cell power supply control circuits 3 and the leak compensation circuits 4 are the same as those described above in Embodiment 1. The components of the redundant circuit block 6 are the same as those of the regular circuit block 5.

Typically, in order to improve the product yield, a semiconductor memory device has a redundant circuit block including a spare memory cell array, in addition to a regular circuit block including a regular memory cell array. During the inspection process of the semiconductor memory device, if it is determined that there is a defective memory cell in a normal memory cell array, the defective memory cell array is replaced by the spare memory cell array, so as to complete the semiconductor memory device as a non-defective product. This is called “defect relief by redundancy”. The semiconductor memory device of the present embodiment includes the redundant circuit block 6 as a redundant circuit block for the regular circuit block 5.

In the semiconductor memory device of the present embodiment, the redundant signal REDEN is controlled to the L level when defect relief is not performed, whereas the redundant signal REDEN is controlled to the H level when defect relief is performed.

First, a case where defect relief is not performed will be discussed. Where defect relief is not performed, the redundant signal REDEN is at the L level, whereby the inverted redundant signal /REDEN being the output of the inverter INV3, which receives the redundant signal REDEN, is at the H level. In the regular circuit block 5, the inverted redundant signal /REDEN is at the H level, whereby the 3-input AND circuits AND5 to AND8, which receive the inverted redundant signal /REDEN being at the H level, are equivalent logics to the 2-input AND circuits AND1 to AND4 of Embodiment 1 shown in FIG. 1. The 3-input NAND circuits NAND5 to NAND6 are equivalent logics to the 2-input NAND circuits NAND1 to NAND2 of Embodiment 1 shown in FIG. 1. The 2-input NAND circuits NAND7 to NAND8 are equivalent logics to the inverters INV1 to INV2 of Embodiment 1 shown in FIG. 1. Moreover, the P-type MOS transistor QP6, which receives the redundant signal REDEN being at the L level, is equivalent in operation to the P-type MOS transistor QP6 of Embodiment 1 shown in FIG. 1.

As described above, it can be seen that where defect relief is not performed, the circuit configuration and the operation of the regular circuit block 5 of FIG. 7 are equal to those of Embodiment 1 shown in FIG. 1.

The redundant circuit block 6 will now be described. Where defect relief is not performed, the redundant circuit block 6 does not need to operate. Since the 3-input AND circuits ANDR5 to ANDR8, which receive the redundant signal REDEN being at the L level, all output the L level, the N-type MOS transistors QNR5 to QNR8, which receive outputs of the 3-input AND circuits ANDR5 to ANDR8, respectively, are always OFF. Therefore, where defect relief is not performed, there is no influence on the redundant bit lines BLR1 to BLR2 and /BLR1 to /BLR2. Since the 3-input NAND circuits NANDR5 to NANDR6, which receive the redundant signal REDEN being at the L level, all output the H level, the P-type MOS transistor QP7, which receives the output of one of the 3-input NAND circuits NANDR5 to NANDR6, is always OFF. The P-type MOS transistor QP6, which receives the inverted redundant signal /REDEN being at the H level, is also always OFF. Therefore, the memory cell power supply control circuits 3 in the redundant circuit block 6 are all inactive in a case where defect relief is not performed. Since the 2-input NAND circuits NANDR7 to NANDR8, which receive the redundant signal REDEN being at the L level, all output the H level, the P-type MOS transistor QP8, which receives the output of one of the 2-input NAND circuits NANDR7 to NANDR8, is also always OFF. Therefore, the leak compensation circuits 4 in the redundant circuit block 6 are all inactive in a case where defect relief is not performed.

As described above, where defect relief is not performed, the memory cell power supply control circuits 3 and the leak compensation circuits 4 in the redundant circuit block 6 of FIG. 7 are all inactive, whereby the redundant memory cell power supply VDDMR1 (VDDMR2) is not supplied. Therefore, power is not supplied to the memory cells 1 in the redundant circuit block 6, and the excessive leak current, or the like, does not flow through the memory cells 1, whereby it is possible to reduce the power consumption of the semiconductor memory device.

Now, a case where defect relief is performed will be discussed. In this case, the redundant circuit block 6 operates, instead of the regular circuit block 5. Where defect relief is performed, the redundant signal REDEN is at the H level, whereby the inverted redundant signal /REDEN being the output of the inverter INV3, which receives the redundant signal REDEN, is at the L level.

In the redundant circuit block 6, the redundant signal REDEN is at the H level, whereby the 3-input AND circuits ANDR5 to ANDR8, which receives the redundant signal REDEN being the H level, are equivalent logics to the 2-input AND circuits AND1 to AND4 of Embodiment 1 shown in FIG. 1. The 3-input NAND circuits NANDR5 to NANDR6 are equivalent logics to the 2-input NAND circuits NAND1 to NAND2 of Embodiment 1 shown in FIG. 1. The 2-input NAND circuits NANDR7 to NANDR8 are equivalent logics to the inverters INV1 to INV2 of Embodiment 1 shown in FIG. 1. The P-type MOS transistor QP6, which receives the inverted redundant signal /REDEN being at the L level, is equivalent in operation to the P-type MOS transistor QP6 of Embodiment 1 shown in FIG. 1.

As described above, it can be seen that where defect relief is performed, the circuit configuration and the operation of the redundant circuit block 6 of FIG. 7 are equal to those of Embodiment 1 shown in FIG. 1.

The regular circuit block 5 will now be described. Where defect relief is performed, the regular circuit block 5 does not need to operate. Since the 3-input AND circuits AND5 to AND8, which receive the inverted redundant signal /REDEN being at the L level, all output the L level, the N-type MOS transistors QN5 to QN8, which receive the 3-input AND circuits AND5 to AND8, respectively, are always OFF. Therefore, where defect relief is performed, there is no influence on the bit lines BL1 to BL2 and /BL1 to /BL2. Since the 3-input NAND circuits NAND5 to NAND6, which receive the inverted redundant signal /REDEN being at the L level, all output the H level, the P-type MOS transistor QP7, which receives the output of one of the 3-input NAND circuits NAND5 to NAND6, is always OFF. The P-type MOS transistor QP6, which receives the redundant signal REDEN being the H level, is also always OFF. Therefore, the memory cell power supply control circuits 3 in the regular circuit block 5 are all inactive in a case where defect relief is performed. Since the 2-input NAND circuits NAND7 to NAND8, which receive the inverted redundant signal /REDEN being at the L level, all output the H level, the P-type MOS transistor QP8, which receives the output of one of the 2-input NAND circuits NAND7 to NAND8, is also always OFF. Therefore, the leak compensation circuits 4 in the regular circuit block 5 are all inactive in a case where defect relief is performed.

As described above, where defect relief is performed, the memory cell power supply control circuits 3 and the leak compensation circuits 4 in the regular circuit block 5 of FIG. 7 are all inactive, whereby the memory cell power supply VDDM1 (VDDM2) is not supplied. Therefore, power is not supplied to the memory cells 1 in the regular circuit block 5, and the excessive leak current, or the like, does not flow through the memory cells 1, whereby it is possible to reduce the power consumption of the semiconductor memory device.

In the description above, the semiconductor memory device of the present invention includes only a small number of memory cells, precharge circuits, memory cell power supply control circuits, leak compensation circuits, etc., as shown in FIG. 1 or FIG. 7, in order to facilitate the understanding of the present invention. However, greater numbers of these components may be provided. It is understood that such a semiconductor memory device with a greater number of these components gives the same effect as those given by the semiconductor memory devices of the embodiments above.

Some of the configurations shown in FIGS. 2 to 6 may be combined with each other, or these configurations may be applied to the configuration with defect relief as shown in FIG. 7, as far as such combinations are logically possible.

As described above, with the semiconductor memory device of the present invention, the memory cell power supply can be optimally controlled according to the arrangement of the memory cell array, the transistor characteristics, the power supply voltage, the temperature condition, etc. Particularly, by improving the write level of the memory cell, it is possible to realize a semiconductor memory device with stable memory cell characteristics with a low power consumption and a small area. Thus, the present invention is useful as a semiconductor memory device including flip-flop-type memory cells, or the like. 

1. A semiconductor memory device comprising: a memory cell including a flip-flop; and a memory cell power supply circuit for supplying a cell power supply voltage to the memory cell, wherein: the memory cell power supply circuit supplies a cell power supply voltage in a first period and a different cell power supply voltage in a second period; the memory cell power supply circuit supplies a predetermined first power supply voltage in a first case where the cell power supply voltage is supplied in a data read cycle and in a second case where data is not written to a memory cell to which the cell power supply voltage is supplied in a write cycle; the memory cell power supply circuit supplies a second power supply voltage lower than the first power supply voltage in a third case where data is written to a memory cell to which the cell power supply voltage is supplied in a write cycle, the memory cell power supply circuit outputs the second power supply voltage by dividing a predetermined voltage; the memory cell power supply circuit includes a first transistor, a second transistor, and a third transistor; the first transistor has a predetermined driving capability, and outputs the first power supply voltage; a voltage divided by the first transistor and the second transistor is output as the second power supply voltage; the first transistor and the second transistor have the same polarity as a load transistor of the flip-flop; and the third transistor supplies the first power supply voltage to the memory cell in the first case and the second case.
 2. The semiconductor memory device of claim 1, wherein: the first power supply voltage is output in response to a write disable control signal or a column non-select signal; and the second power supply voltage is output in response to a write enable control signal and a column select signal.
 3. The semiconductor memory device of claim 1, wherein a voltage divider ratio of the voltage division is determined according to a threshold voltage of a load transistor of the flip-flop.
 4. The semiconductor memory device of claim 1, wherein the memory cell power supply circuit is formed by a source follower circuit, and at least transistors forming the source follower have the same polarity as that of a load transistor of the memory cell.
 5. The semiconductor memory device of claim 1, wherein the second power supply voltage is supplied to a source terminal of a load transistor of the memory cell, and a voltage higher than the second power supply voltage is supplied to a substrate of the load transistor.
 6. The semiconductor memory device of claim 1, further comprising a booster circuit, wherein a boosted voltage is supplied as the first power supply voltage.
 7. The semiconductor memory device of claim 1, wherein a plurality of groups of memory cells are provided, with their transistor threshold voltages differing from one group to another; and a voltage divider ratio of the voltage division used by the memory cell power supply circuit supplying the second power supply voltage to each memory cell group is determined according to the transistor threshold voltage.
 8. The semiconductor memory device of claim 1, wherein a driving capability at the time when the memory cell power supply circuit supplies the second power supply voltage is determined according to a temperature of use of the semiconductor memory device.
 9. The semiconductor memory device of claim 1, wherein a plurality of groups of memory cells are provided, with their transistor threshold voltages differing from one group to another; and a driving capability at the time when the memory cell power supply circuit supplies the second power supply voltage to each memory cell group is determined according to the transistor threshold voltage.
 10. The semiconductor memory device of claim 1, wherein a plurality of groups of memory cells are provided, with their transistor threshold voltages differing from one group to another; and a driving capability at the time when the memory cell power supply circuit supplies the first power supply voltage to each memory cell group is determined according to the transistor threshold voltage.
 11. The semiconductor memory device of claim 1, wherein the memory cell power supply circuit can supply the same cell power supply voltage in the first period and in the second period according to a predetermined control signal.
 12. The semiconductor memory device of claim 1, wherein the memory cell power supply circuit is configured so that a driving capability at the time when the memory cell power supply circuit supplies the first power supply voltage can be switched from one to another according to a predetermined control signal.
 13. The semiconductor memory device of claim 1, wherein: the semiconductor memory device has a function of relieving a defect with redundancy; and in a case where a defect is to be relieved, a supply of a power supply voltage from the memory cell power supply circuit to the memory cell for which a defect is to be relieved is stopped.
 14. The semiconductor memory device of claim 1, wherein: the semiconductor memory device has a function of relieving a defect with redundancy; and in a case where a defect is not relieved, a supply of a power supply voltage from the memory cell power supply circuit to a redundant memory cell, which is not to be used, is stopped.
 15. A semiconductor integrated circuit, comprising a plurality of semiconductor memory devices of claim
 1. 16. A semiconductor memory device comprising: a memory cell including a flip-flop; and a memory cell power supply circuit for supplying a cell power supply voltage to the memory cell, wherein: the memory cell power supply circuit supplies a cell power supply voltage in a first period and a different cell power supply voltage in a second period; the memory cell power supply circuit supplies a predetermined first power supply voltage in a first case where the cell power supply voltage is supplied in a data read cycle and in a second case where data is not written to a memory cell to which the cell power supply voltage is supplied in a write cycle; the memory cell power supply circuit supplies a second power supply voltage lower than the first power supply voltage in a third case where data is written to a memory cell to which the cell power supply voltage is supplied in a write cycle, the memory cell power supply circuit outputs the second power supply voltage by dividing a predetermined voltage; the memory cell power supply circuit includes a first transistor, a second transistor, and a third transistor; the first transistor has a predetermined driving capability, and outputs the first power supply voltage; a voltage divided by the first transistor and the second transistor is output as the second power supply voltage; and the third transistor supplies the first power supply voltage to the memory cell in the first case and the second case. 